In conventional refrigerating machines, air conditioners, cold storage chambers and the like, freon containing fluorine and chlorine was used as a refrigerant. Examples of the freon include chlorofluorocarbon (CFC) such as R-11 (trichloromonofluoromethane) or R-12 (dichlorodifluoromethane), and hydrochlorofluorocarbon (HCFC) such as R-22 (monochlorodifluoromethane).
However, the production and use of the freon have been internationally regulated due to a recent problem of ozone layer depletion, and a novel hydrogen-containing freon refrigerant not containing chlorine is started to be used instead of the conventional freon nowadays. Examples of the hydrogen-containing freon refrigerant include tetrafluoroethane (R-134 or R-134a) and a mixed refrigerant of hydrofluorocarbon (HFC) such as R410A or R407C.
Although the HFC does not deplete the ozone layer, however, it has a high greenhouse effect, and hence is not necessarily an excellent refrigerant from the viewpoint of global warming that has become a recent problem.
Therefore, a lower hydrocarbon having 2 to 4 carbon atoms has been recently gathering attention because it does not deplete the ozone layer and causes an extremely small influence on the global warming as compared with the aforementioned chlorine or non-chlorine fluorocarbon refrigerants, and it is now being examined to be used, as a refrigerant, in a refrigeration system having high cooling efficiency such as a room air conditioner or an industrial refrigerating machine including a compressor, a condenser, a throttle device, an evaporator and the like, which has been developed with the freon refrigerant. Such a hydrocarbon refrigerant has a global warming potential of 1/100 or less and is highly efficient as compared with R401A that is a mixed refrigerant of hydrofluorocarbon (HFC) currently widely used as a refrigerant for a room air conditioner. In particular, there is a possibility that use of propane having 3 carbon atoms (R290) can largely and economically reduce the influence on the global warming without requiring large-scaled design change of an air conditioner. However, this propane is combustible, and therefore, there are a problem of technical development for using it safely, and a problem of selection, as a lubricant, of a refrigerating machine oil having appropriate compatibility with this refrigerant.
As a lubricating oil for the lower hydrocarbon refrigerant, a mineral oil of, for example, naphthene- or paraffin-based mineral oil, an alkylbenzene oil, an ester oil, an ether oil and a fluorinated oil having compatibility with the refrigerant have been proposed. Besides, a polyol ester (POE) has been proposed as the ester oil (see Patent Literature 1).